Add 'funding' field to package.json

This was introduced to `npm` recently and allows users of the ol
package to e.g. type `npm fund ol` and our opencollective page
opens in a browser. Just typing `npm fund` in a npm package will list
the funding options of the dependencies of the current project.

package.json

@@ -35,6 +35,10 @@
   "bugs": {
     "url": "https://github.com/openlayers/openlayers/issues"
   },
+  "funding": {
+    "type": "opencollective",
+    "url": "https://opencollective.com/openlayers"
+  },
   "dependencies": {
     "elm-pep": "^1.0.4",
     "pbf": "3.2.1",